1. Field of the Invention:
This invention relates to a diaphragm for an electro-acoustic transducer, and more particularly, to a diaphragm for use in a moving coil loudspeaker.
2. Description of the Prior Art:
Diaphragms for electro-acoustic transducers are fundamentally required to have a high dynamic modulus, a moderate loss tangent, and a moderate density. They were previously made mainly of paper, but recently thermoplastics films, such as polyolefin, polyester and polyamide films, have come into frequent use, since they provide excellent acoustical properties, have a high degree of moldability, and lend themselves to mass production at a low cost. For example, British Pat. No. 1,563,511 discloses an electro-acoustic transducer diaphragm made of polyolefin film.
To improve acoustical properties, efforts have been made to develop a diaphragm having a higher dynamic modulus. A known method of improving the dynamic modulus of a polymeric material is incorporation of a reinforcing filler. If a fibrous reinforcer, such as glass or carbon fibers is used, however, an anisotropic diaphragm is formed because of the orientation of fibers that takes place during the extrusion forming of the diaphragm. If a flaky reinforcer, such as graphite or seashell powder, is employed, it is difficult to obtain a diaphragm having a satisfactorily improved dynamic modulus. Laid-Open Japanese Patent Specification No. 162695/1980 discloses such a diaphragm for an electro-acoustic transducer that is formed from a thermoplastic resin and flaky graphite. The use of mica for making a diaphragm for an electro-acoustic transducer is also known. Laid-Open Japanese Patent Specification No. 47816/1978 discloses a diaphragm formed by a papermaking machine from a mixture of cellulose fibers and mica dispersed in water, and Laid-Open Japanese Patent Specification No. 75316/1977 discloses a diaphragm formed by a papermaking machine from a mixture of carbon fibers and mica. These diaphragms have, however, not met any success in practice since mica, which inherently does not have any entangling property, is difficult to handle with a papermaking machine. The formation of a loudspeaker diaphragm from a sheet made of a mixture of polyvinyl chloride and mica is also known, as disclosed in Laid-Open Japanese Patent Specification No. 136796/1980, but no diaphragm having satisfactory acoustical properties as proposed therein has actually been obtained.